1. Field of the Invention
The present invention relates to a method and apparatus for detecting a mark position wherein a mark is scanned by a beam, such as a laser beam, and the beam reflected by the mark is used to detect the position of the mark.
2. Description of the Prior Art
As an example of apparatus wherein a position of a mark is detected, an exposure and alignment apparatus for manufacturing integrated circuits contains a laser beam scanner which scans the alignment marks pre-formed on the mask and wafer, and the light scattered and diffracted by edges of the marks is used for detecting the mark positions so as to provide signals for alignment between the wafer and mask. More particularly, the light specularly reflected by the mask and wafer is removed, and only the scattered and diffracted light is received by a photoelectric transducer which produces an electric pulse signal in response to the quantity of the scattered and diffracted light. Then, the intervals between the pulses are measured by a clock pulse counter or the like to determine the position of the mark.
FIG. 1 shows the principle of the detection of the mark positions in such prior art, wherein the mask has such an alignment mark M as shown in FIG. 1(a), while the wafer has such an alignment mark W as shown in FIG. 1(b). Those marks are overlaid as shown in FIG. 1(c). Then, a laser beam having a relatively large diameter in cross-section scans the alignment marks M and W along a scanning line l, so that a photoelectric transducer receiving the light reflected by the marks produces the pulse signals as shown in FIG. 1(d). The signals are cut at a suitable threshold voltage V with the use of a comparator, whereby a series of rectangular pulse signals are obtained. The spacings or intervals between the adjacent marks are calculated from the series of pulses so that the relative position between the alignment marks M and W is discriminated. The relative position between the mask and wafer is changed in response to the discrimination indicative of the amount of misalignment, by suitable means to bring them into alignment.
However, the inventor has found that, in this system, the relation between the actual mark position and the position where the pulse signal is generated is not constant but varies for each of the scans, so that the correct position detection is difficult.